This invention relates to an apparatus and method for measuring strain on a structural surface. More particularly, the invention relates to a fiber optic sensing system attachable to a surface where strain is to be measured and adapted to attenuate the light signal transmitted through a curved portion of fiber optic material in relation to deformation of a curved portion in response to strain applied to the surface.
Optical fibers in general are known in the art, and typically comprise a transparent core of a suitable glass or plastic material which is carried within a relatively thin cylindrical cladding having an index of refraction less than the refractive index of the core. When a light signal such as a collimated beam generated by a laser is focused upon one end of the fiber, the fiber core functions as a waveguide to transmit or propagate the light signal through the core with relatively small internal intensity losses or transmission of the signal to the cladding.
A fiber optic transducer for measuring strain or pressure has an advantage over a mechanical or electrical transducer in that it is not disturbed by electromagnetic interference at other than optical frequencies. This is particularly desirable for making measurements in electromagnetically noisy environments such as various points inside a jet engine, in electrical transmission and distribution equipment, or other environments where strong electric and/or magnetic fields may be present. Fiber optic sensors may also be advantageously formed as an integral portion of fiberglass composite structures and used to measure the structural integrity or loads applied thereto. Such fiber optic detectors are economical and durable, and introduce a minimum of additional weight to the structure.
Optical fibers of the type utilized in the present invention are characterized in that straight portions or portions having gradual bends in the fiber have little or no effect upon transmission of the light signal, thereby permitting low loss transmission through the fiber for emission at the opposite end of the fiber regardless of the number of gradual bends and turns. Practical applications of such light conductive fibers have included, for example, devices to illuminate or to permit viewing of inaccessible areas, such as areas inside the human body, or as a telecommunications link wherein the light signal is modulated to represent information that can be detected and demodulated at the opposite end of the fiber.
It has been recognized, however, that relatively sharp bends in such optical fibers can have a significant effect upon the transmissivity of the fiber core. More specifically, the presence of a sharp bend having a bend diameter on the order of a few millimeters, commonly referred to as a microbend, results in substantial attenuation of the propagated light signal. That attenuation results from scattering of a portion of the signal from the fiber core to the cladding. Most of the scattered light portion is ultimately lost to the surrounding environment. The microbending consequently causes a detectable attenuation of a light signal passing through the fiber, wherein the degree of attenuation is indicative of the curvature of the fiber.
Contemporary optical strain measurement systems include one or more optical fibers fixed to the surface where strain is to be measured. The output of such systems is typically analyzed to detect an applied load by monitoring phase velocity, crosstalk, light intensity, or interference between adjacent fibers. Such systems do not include an optical transducer, as disclosed below, designed to interact with an applied optical signal such that the attenuation of the optical interrogating signal is made to be a linear function of the applied load. Additionally, none of those systems include a load measurement system wherein the applied load may be measured by linear scaling of irregularities in an optical signal received from a fiber optic sensor. Consequently, such contemporary optical systems typically require complex processing to resolve the applied strain and may be unsuitable for application wherein the interrogating signal is modulated.
Moreover, contemporary optical measurement systems are not believed to disclose or suggest a load measurement system wherein a predetermined number of interrogating pulses of substantially identical form are progressively directed to each of a plurality of optical transducers disposed in parallal along a common optical bus.